The interest in developing biodegradable lubricants for use in applications which result in the dispersion of such lubricants into waterways, such as rivers, oceans and lakes, has generated substantial interest by both the environmental community and lubricant manufacturers. The synthesis of a lubricant which maintains its cold-flow properties and additive solubility without loss of biodegradation or lubrication would be highly desirable.
Basestocks for biodegradable two-cycle lubricants should typically meet five criteria: (1) solubility with dispersants and other additives such as polyamides; (2) good cold flow properties (such as, less than -40.degree. C. pour point and less than 7500 cps at -25.degree. C.); (3) sufficient biodegradability to off-set the low biodegradability of any dispersants and/or other additives to the formulated lubricant; (4) good lubricity without the aid of wear additives; and (5) very low toxicity of greater than 1,000 ppm.
The Organization for Economic Cooperation and Development (OECD) issued draft test guidelines for degradation and accumulation testing in December 1979. The Expert Group recommended that the following tests should be used to determine the "ready biodegradability" of organic chemicals: Modified OECD Screening Test, Modified MITI Test (I), Closed Bottle Test, Modified Sturm Test, the Modified AFNOR Test, and the Manometric Respirometer Test. The Group also recommended that the following "pass levels" of biodegradation, obtained within 28 days, may be regarded as good evidence of "ready biodegradability": (Dissolved Organic Carbon (DOC)) 70%; (Biological Oxygen Demand (BOD)) 60%; (Total Organic Carbon (TOD)) 60%; (CO.sub.2) 60%; (DOC) 70%; and (O.sub.2 consumption) 60%, respectively, for the tests listed above. Therefore, the "pass level" of biodegradation, obtained within 28 days, using the Modified Sturm Test is at least (CO.sub.2) 60% and the Manometric Respirometer is at least (O.sub.2) 60%.
The OECD guideline for testing the "ready biodegradability" of chemicals under the Modified Sturm test (OECD 301B, adopted May 12, 1981, and which is incorporated herein by reference) involves the measurement of the amount of CO.sub.2 produced by the microorganisms during the degradation of the test compound which is measured and expressed as a percent of the theoretical CO.sub.2 (ThCO.sub.2) it should have produced calculated from the carbon content of the test compound. Biodegradability is therefore expressed as a percentage of ThCO.sub.2. The Modified Sturm test is run by spiking a chemically defined liquid medium, essentially free of other organic carbon sources, with the test material and inoculated with sewage micro-organisms. The CO.sub.2 released is trapped as BaCO.sub.3. After reference to suitable blank controls, the total amount of CO.sub.2 produced by the test compound is determined for the test period and calculated as the percentage of total CO.sub.2 that the test material could have theoretically produced based on carbon composition. See G. van der Waal and D. Kenbeek, "Testing, Application, and Future Development of Environmentally Friendly Ester Based Fluids", Journal of Synthetic Lubrication, Vol. 10, Issue No. 1, April 1993, pp. 67-83, which is incorporated herein by reference.
The OECD guideline for testing the "ready biodegradability" of chemicals under the Manometric Respirometer test (OECD 301F, adopted Jul. 17, 1992, and which is incorporated herein by reference) involves the measurement of the amount of O.sub.2 consumed by the microorganisms during the biodegradation of the test compound. It is measured and expressed as a percent of the theoretical O.sub.2 demand (ThOD) it should have consumed calculated from the carbon content of the test compound. Biodegradability is therefore expressed as a percentage of ThOD. The Manometric Respirometer test is run by spiking a chemically defined liquid medium, essentially free of other organic carbon sources, with the test material and inoculated with sewage microorganisms. The oxygen consumed is determined either by measuring the amount of oxygen (produced electrolytically) required to maintain constant gas volume in the respirometer flask, or from the change in volume or pressure (or a combination of the two) in the apparatus. After reference to suitable blank controls, the total amount of oxygen consumed by the microorganisms is determined for the test period and calculated as the percentage of total oxygen demand that the microorganisms would have theoretically required to biodegrade the test compound based on carbon composition. See "OECD Guidelines for the Testing of Chemicals", Vol. 1, OECD 1993.
One basestock in current use today is rapeseed oil (i.e., a triglyceride of fatty acids, e.g., 7% saturated C.sub.12 to C.sub.18 acids, 50% oleic acid, 36% linoleic acid and 7% linolenic acid), having the following properties: a viscosity at 40.degree. C. of 47.8 cSt, a pour point of 0.degree. C., a flash point of 162.degree. C. and a biodegradability of 85% by the Modified Sturm test. Although it has very good biodegradability, its use in biodegradable lubricant applications is limited due to its poor low temperature properties and poor stability.
Unless they are sufficiently low in molecular weight, esters synthesized from both linear acids and linear alcohols tend to have poor low temperature properties. Even when synthesized from linear acids and highly branched alcohols, such as polyol esters of linear acids, high viscosity esters with good low temperature properties can be difficult to achieve. In addition, pentaerythritol esters of linear acids exhibit poor solubility with dispersants such as polyamides, and trimethylolpropane esters of low molecular weight (i.e., having a carbon number less than 14) linear acids do not provide sufficient lubricity. This lower quality of lubricity is also seen with adipate esters of branched alcohols. Since low molecular weight linear esters also have low viscosities, some degree of branching is required to build viscosity while maintaining good cold flow properties. When both the alcohol and acid portions of the ester are highly branched, however, such as with the case of polyol esters of highly branched oxo acids, the resulting molecule tends to exhibit poor biodegradation as measured by the Modified Sturm test (OECD Test No. 301B).
In an article by Randles and Wright, "Environmentally Considerate Ester Lubricants for the Automotive and Engineering Industries", Journal of Synthetic Lubrication, Vol. 9-2, pp. 145-161, it was stated that the main features which slow or reduce microbial breakdown are the extent of branching, which reduces .beta.-oxidation, and the degree to which ester hydrolysis is inhibited. The negative effect on biodegradability due to branching along the carbon chain is further discussed in a book by R. D. Swisher, "Surfactant Biodegradation", Marcel Dekker, Inc., Second Edition, 1987, pp. 415-417. In his book, Swisher stated that "The results clearly showed increased resistance to biodegradation with increased branching . . . . Although the effect of a single methyl branch in an otherwise linear molecule is barely noticeable, increased resistance to biodegradation! with increased branching is generally observed, and resistance becomes exceptionally great when quaternary branching occurs at all chain ends in the molecule." The negative effect of alkyl branching on biodegradability was also discussed in an article by N. S. Battersby, S. E. Pack, and R. J. Watkinson, "A Correlation Between the Biodegradability of Oil Products in the CEC-L-33-T-82 and Modified Sturm Tests", Chemosphere, 24(12), pp. 1989-2000 (1992).
Initially, the poor biodegradation of branched polyol esters was believed to be a consequence of the branching and, to a lesser extent, to the insolubility of the molecule in water. However, recent work by the present inventors has shown that the non-biodegradability of these branched esters is more a function of steric hindrance than of the micro-organism's inability to breakdown the tertiary and quaternary carbons. Thus, by relieving the steric hindrance around the ester linkage(s), biodegradation can more readily occur with branched esters.
Branched synthetic polyol esters have been used extensively in non-biodegradable applications, such as refrigeration lubricant applications, and have proven to be quite effective if 3,5,5-trimethylhexanoic acid is incorporated into the molecule at 25 molar percent or greater. However, trimethylhexanoic acid is not biodegradable as determined by the Modified Sturm test (OECD 301B), and the incorporation of 3,5,5-trimethylhexanoic acid, even at 25 molar percent, would drastically lower the biodegradation of the polyol ester due to the quaternary carbons contained therein and the resulting steric hindrance that the branching would cause.
Likewise, incorporation of trialkyl acetic acids (i.e., neo acids) into a polyol ester produces very useful refrigeration lubricants. These acids do not, however, biodegrade as determined by the Modified Sturm test (OECD 301B) and cannot be used to produce polyol esters for biodegradable applications. Polyol esters of all branched acids can be used as refrigeration oils as well. However, they do not rapidly biodegrade as determined by the Modified Sturm Test (OECD 301B) and, therefore, are not desirable for use in biodegradable applications.
Although polyol esters made from purely linear C.sub.5 and C.sub.10 acids for refrigeration applications would be biodegradable under the Modified Sturm test, they would not work as a lubricant in two-cycle engine applications because their viscosities would be too low and wear additives would be needed. It is extremely difficult to develop a lubricant basestock which is capable of exhibiting all of the various properties required for biodegradable lubricant applications, i.e., high viscosity, low pour point, oxidative stability and biodegradability as measured by the Modified Sturm test.
U.S. Pat. No. 4,826,633 (Carr et al.), which issued on May 2, 1989, discloses a synthetic ester lubricant basestock formed by reacting at least one of trimethylolpropane and monopentaerythritol with a mixture of aliphatic monocarboxylic acids. The mixture of acids includes straight-chain acids having from 5 to 10 carbon atoms and an iso-acid having from 6 to 10 carbon atoms, preferably iso-nonanoic acid (i.e., 3,5,5-trimethylhexanoic acid). This basestock is mixed with a conventional ester lubricant additive package to form a lubricant having a viscosity at 99.degree. C. (210.degree. F.) of at least 5.0 centistokes and a pour point of at least as low as -54.degree. C. (-65.degree. F.). This lubricant is particularly useful in gas turbine engines. The Carr et al. patent differs from the present invention for two reasons. Firstly, it preferably uses as its branched acid 3,5,5-trimethylhexanoic acid which contains a quaternary carbon in every acid molecule. The incorporation of quaternary carbons within the 3,5,5-trimethylhexanoic acid inhibits biodegradation of the polyol ester product. Also, the lubricant according to Carr et al. exhibits high oxidative stability, as measured by a high pressure differential scanning calorimeter (HPDSC), i.e., about 35 to 65 minutes. This high stability is a result of the quaternary branching which increases the number of primary hydrogens (most stable) and decreases the number of secondary and tertiary hydrogens (less stable). The quaternary branching further increases stability by shielding the molecule (through steric hindrance) from attack by free radicals. However, the quaternary branching also shields the ester linkage making it difficult to impossible for microorganisms to attack the ester linkage, resulting in poor biodegradation. Conversely, the lubricant according to the present invention is lower in stability, i.e., it has a HPDSC reading of about 12-17 minutes. One reason for the lower stability is the fact that no more than 10% of the branched acids used to form the lubricant's ester basestock contain a quaternary carbon. The absence of quaternary carbons allows the micro-organisms to first attack the ester linkage and then the carbon-to-carbon bonds of the alcohol and acid moieties and effectively cause the ester to biodegrade.
The present inventors have discovered that blends of natural and synthetic lubricant basestocks with high viscosity complex alcohol esters unexpectedly provide a lubricating basestock having the following desirable properties: biodegradability, wide range of viscosities, low acid content, good pour point, excellent lubricity, seal compatibility, and low toxicity.
With the right ratios of polyol to polybasic acid to monohydric alcohol, complex alcohol esters can be produced which have reduced cost (approximately half the cost of complex acid esters), high viscosity (greater than 100 cSt at 40.degree. C.), good thermal and oxidative stability, good biodegradability, low toxicity, good low temperature properties, and excellent lubricity. When blended with lower viscosity oils, a wide range of iso grade products can be produced which meet stringent end-use specifications. The present inventors have discovered that when the amount of linear monohydric alcohol exceeds 20% of the total alcohol used, then the pour point is too high, e.g., above -30.degree. C. Furthermore, the present inventors have discovered that the ratio of polybasic acid to polyol is critical in the formation of a complex alcohol ester. That is, if this ratio is too low then a complex alcohol ester contains undesirable amounts of heavies which reduces biodegradability and increases the hydroxyl number of the ester which increases the corrosive nature of the resultant ester which is also undesirable. If, however, the ratio is too high then the resultant complex alcohol ester will have an undesirably low viscosity (reducing its applicability in certain iso grade applications) and poor seal swell characteristics.
The present inventors have also discovered that the ratio of the monohydric alcohol to polybasic acid is equally critical in the formation of complex alcohol esters. That is, if this ratio is too low then a complex alcohol ester contains undesirable amounts of heavies due to increased cross-linking which reduces biodegradation. It also increases the total acid number of the ester which increases the corrosive nature of the resultant ester and catalyzes the hydrolysis of the ester in the presence of water, both of which are undesirable. If, however, the ratio is too high, transesterification occurs producing more diester. The resultant complex alcohol ester will have an undesirably low viscosity (reducing its applicability in certain iso grade applications) and poor seal swell characteristics.
Other conventional natural and synthetic esters may each provide one or more of the desired attributes, e.g., high viscosity, good low temperature properties, biodegradability, lubricity, seal compatibility, low toxicity, and good thermal and oxidative stability, but none appears to be able to meet all of the product attributes by themselves. For example, some synthetic esters are capable of meeting the high viscosity property, but fail the biodegradability, low temperature requirements, or low toxicity requirements. Similarly, the natural basestocks such as rapeseed oil are capable of meeting the biodegradability and toxicity properties, but fail to meet the required high viscosity, lubricity, and thermal and oxidative stability properties.
The blended lubricant basestocks according to the present invention comprise a complex alcohol ester and at least one additional natural, hydrocarbon-based and/or synthetic basestock. These blends appear to satisfy all of the desired attributes for fully formulated two-cycle lubricant basestocks by providing the basestock with a unique level of biodegradability in conjunction with effective lubricating properties. They also provide excellent thermal and oxidative stability, good low temperature properties (i.e., low pour points), low toxicity, low volatility, and good seal compatibility.
Moreover, the present inventors have demonstrated that an unexpected, synergistic effect occurs when the complex alcohol esters of the present invention are blended with either a natural, hydrocarbon-based and/or synthetic ester basestock, i.e., the blended basestock unexpectedly exhibits enhanced product attributes versus either the complex alcohol ester or other basestock by itself. Thus, the blended basestocks according to the present invention exhibit the following attributes: excellent lubricity, seal compatibility, biodegradability, low toxicity, good low temperature properties, a wide viscosity range to meet various iso grade needs, good thermal and oxidative stability, and improved engine performance.